An analog-to-digital converter is a circuit which converts a physical analog signal (continuous in value and continuous in time) to a digital signal (discrete in value and discrete in time) which is suited for use in a digital computer. A digital-to-analog converter performs the reverse process of converting a digital signal into a physical analog signal. Most commonly, the analog signals of interest are voltage waveforms which might be received from an electromechanical transducer, such as a microphone, or used to drive an electromechanical transducer, such as a loudspeaker. The digital signal is a series of numeric values which correspond to samples of the voltage waveform taken at discrete points in time. These numeric values may correspond directly to a physical voltage, in units of volts, or they may be in arbitrary units.
Most commonly, analog-to-digital and digital-to-analog converters are ratiometric, meaning that the digital output, in the case of an analog- to-digital converter, or the digital input, in the case of a digital-to-analog converter, corresponds to some fraction of an arbitrary reference voltage. For example, an analog-to-digital converter for digital audio applications might produce a 16-bit, 2's complement digital signal where a positive full scale digital output corresponds to an input of +2 V, and a negative full scale output corresponds to an input of -2 V. In many commercial parts, the reference level for the analog-to-digital or digital-to-analog conversion is determined by an on-chip voltage reference circuit. Ideally, the voltage reference circuitry will produce a reference level which is precise, stable with temperature, and independent of the actual power supply voltage seen by the part. In other commercial parts, the voltage reference is provided by the user on an external pin. In many applications this is undesirable as the user is forced to generate a precise voltage reference using additional components.
Ratiometric converters which use either an external voltage reference, or which divide down the power supply for use as the reference, do have one advantage over converters with internal voltage references in that they can be readily designed to operate effectively over a wide range of power supply voltages. For example, one might design a converter which can operate with a total supply voltage of anywhere from 10 V to 30 V, and where the external voltage reference can be scaled with the power supply to make effective use of the available supply voltage. So, a user might operate the converter with supply voltages of +5 V and -5 V, with an external voltage reference of 3 V (realizing a conversion range of -3 V to +3 V). Or, a user could operate the converter with supply voltages of +15 V and -15 V, with an external voltage reference of 10 V, thereby realizing a conversion range of -10 V to +10 V. In contrast, given a fixed, internal voltage reference of 3 V, the same part would always provide a conversion range of -3 V to +3 V, independent of supply voltage.
It can therefore be appreciated that an analog-to-digital converter or digital-to-analog converter with an internal reference voltage which can operate with a multiplicity of supply voltage is desirable.